This is an intermediate to advanced problem, but possible to resolve.
On Mac OS X and Unix, thread support is enabled by adding the -thread option when running the configure script. On Unix platforms where multithreaded programs must be linked in special ways, such as with a special libc, installation will create a separate library, libqt-mt and hence threaded programs must be linked against this library (with -lqt-mt) rather than the standard Qt library.
On both platforms, you should compile with the macro QT_THREAD_SUPPORT defined (e.g. compile with -DQT_THREAD_SUPPORT). On Windows, this is usually done by an entry in qconfig.h.